D.c. motor speed control

ABSTRACT

A speed control for a D.C. motor has a resistance connectable in series with an energy source and the motor armature for starting the motor, a series of voltage dropping diodes connectable in parallel with the resistance for slow speed operation, and switching connections for effectively removing the resistance and the diodes from the armature circuit for high speed operation. Dynamic braking of the motor is also provided when the motor armature is disconnected from the energy source.

United States Patent Woloszyk Feb. 12, 1974 D.C. MOTOR SPEED CONTROL[75] Inventor: Leonard T. woloszyk, Greendale, pmfqary Emm" lerBemardGllheany W is. Assistant ExammerThomas Langer Attorney, Agent, orFirmArthur H. Seidel; Barry E. [73] Assignee: Lakeside ManufacturingInc., Sammons Milwaukee, Wis.

[22] Filed: Feb. 23, 1973 [57] ABSTRACT [21 Appl. No.: 335,054 A speedcontrol for a D.C. motor has a resistance connectable in series with anenergy source and the motor 52 us. Cl 318/139, 318/305, 318/348 9 asane? 0f 5 1 Int Cl p 5/06 droppmg dlOdCS connectable 1n parallel wlththe resls- [58] Field of Search 318/139, 269, 258, 305, 343, f for SlowP l and swflchmg tions for effecttvely removmg the reslstance and the318/347, 348, 379, 380, 418

, diodes from the armature c1rcu1t for h1gh speed opera- ReferencesCited non. Dynamlc braking of the motor is also provided when the motorarmature 1s dlsconnected from the en- UNITED STATES PATENTS ergy Source2,712,625 7/1955 Blitz 318/347 t 2,757,329 7/1956 Lichtenfels 318/380 10Claims 3 Drawmg Flgures PATENTEI] FEB I 2 IBM mDGEOP SPEED D.C. MOTORSPEED CONTROL BACKGROUND OF THE INVENTION The present invention relatesto a speed control for a DC. motor that is suitable for the operation ofan electrically powered wheelchair and like devices.

There are a variety of methods for controlling the speed of a DC. motor.Basically, such methods consist of either (1) changing the fieldstrength of the motor, (2) varying the armature voltage, (3) gating theamount of armature current by SCR control, (4) changing the effectivebrush position, or (5) varying a resistance in the armature circuit. ForDC. motors of a size such as used in driving wheelchairs it would beadvantageous to use the latter method of varying armature circuitresistance. A wheelchair need not require more than two or three stepsof speed control, and this could be achieved by switching values offixed resistance into and out of the armature circuit. A resistanceabsorbs a part of the source voltage and thus effectively lowers thearmature voltage, with a resultant reduction in motor speed for a givenload condition. This methodof controlling motor speed by varyingarmature circuit resistance permits the utilization of a relativelyinexpensive permanent magnet, fixed field excitation type motor, and thearmature voltage may be supplied from a standard energy cell. Althoughthe simple insertion of a fixed resistance into an armature circuit iseffective to produce a reduced speed, it also has the disadvantage ofdecreasing maximum available motor torque. Thus, a motor for driving awheelchair that has an appreciable resistance in its armature circuitmay stall, or never turn over, under a relatively large load, such as isencountered on a deep pile carpeting or in ascending an inclinedsurface.

The problem in operating a wheelchair is that the load to be propelledcan vary over a wide range. The occupant may be small or large. Thechair may be called upon to start-up on a downgrade requiring littleaccelerating force, or conversely on an upward incline requiring muchgreater initial torque. Then too, the surface condition over which thewheelchair rides can materially alter the driving torque requirements.Successful adaptation of a speed control utilizing resistances of afixed value in the motor armature circuit becomes difficult, if a simpleone or two step speed control arrangement is to be utilized. It is asolution of this problem to which the disclosure herein is directed.

SUMMARY OF THE INVENTION The present invention resides in a speedcontrol for a D. C. motor having a resistive element that is connectedin series with the motor armature upon actuation of accompanying controlswitching, a voltage absorbing diode string connected in shunt relationto the resistive element upon further actuation of the accompanyingcontrol switching, and a direct connection between the motor armatureand an energy source upon yet further actuation of the controlswitching.

The invention modifies the method of DC. motor control based onvariation in armature circuit resistance. For starting, a resistance isinserted in the armature circuit by the initial manual movement of aspeed control member. Current inrush, before motor counter electromotiveforce is developed, is limited by the resistance and this eliminates anysudden application of excessive starting torque which might otherwisecause an objectionable abrupt start. Such a control is essential for awheelchair, where the user may be of infirm age or an invalid that needsa feeling of security in operating the chair. The problem of theresistance limiting total available torque, such that for large loadsstart-up or speed cannot be initiated or maintained, is overcome byshunting the resistance with diodes. These diodes are brought intoshunting relation by a continued motion of the manual speed controlmember. They function to increase the current available to the motorarmature. The characteristic of the control circuit now makes moretorque available for starting or for maintaining movement, but with atorque-speed characteristic that the higher speeds are not attainedafter the inertia of the load is overcome.

For higher speed, the manual speed control member can be fully advancedto place the motor armature directly across the voltage source. Thecontrol circuit can also include additional switching for placing theresistance across the motor armature when the motor is disconnected fromthe voltage source. Dynamic braking is thus attained by returning thespeed control member back to zero speed, or stop, position. A smooth,prompt deceleration occurs which gives the user of the wheelcir a securefeeling of control over the chair, while not jerking, or unduly makingan abrupt halt. Also, the chair will not run away on an incline before amanual parking brake can be set. It is a particular discovery that forwheelchair operation the resistance for obtain ing low speed alsofunctions well as the dynamic braking resistance.

It is an object of the invention to provide a speed control for a DC.motor that provides a reduction in motor speed without severely limitingavailable motor torque.

Another object of the invention is to provide a steptype speed controlfor a DC. motor inserting a fixed resistance into the armature circuitthat is inexpensive, simple in design and reliable in operation.

Another object of the invention is to provide a speed control for a DC.motor that furnishes a smooth operation satisfactory for a wheelchair.

Another object of the invention is to provide a speed control for a DC.motor that is actuable through a series of stepped switching functions.

The foregoing and other objects and advantages of the invention willappear from the following description. In the description reference ismade to the accompanying drawing, which forms a part hereof, and inwhich there is shown by way of illustration, and not of limitation, aspecific form in which the invention may be embodied. Such embodimentdoes not represent the full scope of the invention, but rather theinvention may be employed in a variety of embodiments, and reference ismade to the claims herein for interpreting the breadth of the invention.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is a view in perspective of amotorized wheelchair utilizing a motor speed control embodying thepresent invention,

FIG. 2 is a schematic wiring diagram of the motor speed controlembodying the present invention which includes a showing of a controlbox and operating handle in vertical section, and

FIG. 3 is a graphic representation of the speed-torque operation curvesof a DC. motor controlled by the motor speed control.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows a motorizedwheelchair 1 that is more fully described in co-pending United Statesapplication for Letters Patent Ser. No. 281,081, filed Aug. 16, 1972 andentitled Self-Propelled Wheelchair. The wheelchair 1 has a vertical,pivotally mounted steering shaft 2 located at the front and on the leftside of the vehicle. A steerable power unit 3 is mounted on the lowerend of the shaft 2 which includes a drive wheel 4 driven through a chain5 by a DC. motor 6 having a 7 fixed, permanent magnet field. Alightweight cover 7 normally shrouds the power unit 3, and a storagebattery housed in a container 8 provides an energy source for the motor6.

The steering shaft 2 turns rearwardly at its upper end to present atiller arm 9 for turning the shaft 2 and the steerable power unit 3. Thetiller arm 9 terminates in a control box 10, which is shown on anenlarged scale in FIG. 2. The box 10 has a removable cover 11 whichmounts a vertical steering handle 12. The handle 12 is hollow, andsurrounds a vertically depressible, manually operable speed controlmember 13. The rod-like control member 13 is biased upward by a spring14 to the position shown in FIG. 2.

Referring specifically to FIG. 2, the control box 10 houses a set ofthree speed selector switches 15, 16 and 17 tiered one above the other.Roller type operating heads of the switches -17 extend into the downwardpath of travel of the speed control member 13, which may be depressed sothat its lower end successively moves against the switch operatingheads. The switches 15-17 form a portion of the speed control circuit ofthe present invention, and their actuation determines the speed at whichthe wheelchair 1 will be driven by the motor 6.

A pivoted lever 18 is mounted inside the control box 10 that has a lowertip 19. This tip 19 may be swung into and out of the vertical path oftravel of the rod-like control member 13 by actuation of a finger 20 atthe top of the lever 18. Access to the finger 20, for moving the lever18, is afforded by removal of the cover 11, and when the lever tip 19 isin the position shown in FIG. 2 the rod-like speed control member 13 isblocked from operation of the lowermost speed selector switch 17.

Referring further to the schematic wiring diagram of FIG. 2, a standard12-volt battery, or energy cell, contained in the battery case of FIG.1, is referred to by the numeral 21. The positive side of the battery 21connects through a circuit breaker 22 to a normally open contact 23 ofthe first speed selector switch 15. A normally closed contact 24 of theswitch 15 connects through a lead 25 to the negative side of the battery21, and a lead 26 connects from the negative battery side to a doublethrow reversing switch 27 mounted within the control box 10. As shown inboth FIGS. 1 and 2, a manually operable lever 28 of the reversing switch27 protrudes through a side wall of the control box 10 in a positionconvenient to the user of the wheelchair l.

A movable contact 29 of the first speed selector switch 15 is connectedto one end of a resistor 30, and

the opposite end of the resistor 30 connects through a lead 31 to thereversing switch 27.

A normally open contact 32 of the second speed selector switch 16 isjoined to the movable contact 29 of the first switch 15, and the movablecontact 33 of the second switch 16 is connected to one end of a stringof five diodes 34 connected in series. The other end of the string ofdiodes 34 is connected to the lead 31. A normally open contact 35 of thethird speed selector switch 17 is connected to both the open contact 32of the second switch 16 and the movable contact 29 of the first switch15. A movable contact 36 of the third switch 17 is joined through ashorting lead 40 to the lead 31.

The reversing switch 27 has two output leads 37 and 38 that connect tothe armature 39 of the motor 6. By operation of the switch 27 thedirection of rotation of the motor 6 may be reversed, and by locatingthe switch 27 in electrical succession to the motor the polarities ofthe leads 26 and 31"and the remainder of the circuit are retained thesame at all times. Thus, the positive terminal of the battery 21 ispermanently connected to the anodes of the string of diodes 34regardless of the direction of motor rotation.

For operation and control of the motor 6, the steering handle 12 isdesigned to be grasped with the palm and fingers of one hand, leavingthe thumb free to depress the rod-like control member 13. As the member13 is depressed, it successively engages the operating heads of theswitches 15-17, furnishing a three stage speed control that consists ofa first start stage, a second low speed stage and a third high speedstage. The member 13 may be depressed incrementally, to allow the motor6 to operate in any selected one of these three speed control stages, orthe member 13 may be rapidly depressed to the particular stage ofcontrol desired. Upon an initial incremental depression of the member13, it engages the operating head of the upper, or first, speed selectorswitch 15 toopen the normally closed contact 24 and close the normallyopen contact 23. This closure of the-contact 23 connects the resistor 30in series with the motor armature 39 and the battery 21. Armaturecurrent begins flowing from the battery 21 through the resistor 30 andthe armature 39, and the intial, transient armature current is limitedby the resistor 30 before a counter electromotive force is establishedin the armature 39.

A voltage drop is produced across the resistor 30 by the flow ofarmature current, reducing the amount of the available voltage suppliedby the battery 21 that is applied across the armature 39. Consequently,the motor torque developed is less than if full battery voltage wereapplied directly across the motor armature 39. The speed-torquecharacteristic of the motor 6 for this condition is graphicallyrepresented in FIG. 3 by the curve drawn between points A and B, aportion of which is shown as a dashed line for reasons that will becomeapparent. The value of the resistor 30 is chosen to produce a sufficientvoltage drop so that the wheelchair will run in the startstage at arelatively low speed, but with sufficient torque for usual loadconditions. Also, initial acceleration of the wheelchair 1 will be loweror softer for the comfort of the user, than if full battery voltage wereapplied across the motor armature 39.

For usual operation of the wheelchair 1 it is not intended that it beoperated for any length of time in the start stage, because with theresistor 30 in the armature circuit the motor 6 can produce only amodest amount of torque. This is apparent from a comparison of curve A-Bin FIG. 3 with the curve A-C, which depicts the motor speed-torquecharacteristic without the resistor 30 in the armature circuit. Forcurve A-C the motor armature 39 is connected directly across the battery21, and by comparing curves A-B and A-C it is apparent that theinsertion of the resistor 30 in the armature circuit substantiallyreduces the producible torque. For a load requiring a torque greaterthan at the point B, themotor 6 will not turn over when the resistor 30is in the circuit, and for a given load less than the value at B themotor 6 will operate at a much slower speed when the resistor 30 is inthe circuit, than when it is out of the circuit. This is illustrated bythe points D and E on the graphs A-B and A-C respectively, which pointstypify the motor speed when the wheelchair l is driven on a level,substantially smooth surface with a normal person.

The large reduction in the producible motor torque in the start stagemay result in an unsatisfactory wheelchair operation if it is driven onheavy carpeting or up a sharp incline, or is carrying an unusually heavyperson. The motor 6 may stall out under such loading, or never initiatemovement. Thus, a control circuit with a voltage absorbing resistorforobtaining desired startup for normal loads does not serve as a normalwheelchair running stage.

To energize the motor 6 to the low speed stage, the rod-like speedcontrol member 13 is depressed to operate the normally open switch 16for closing the contacts 32, 33. This connects the string of diodes 34in parallel with the resistor30. The diode 34 now act as a voltageresponsive gating device that does not conduct current until it issufficiently forward biased. The number of diodes 34 is specificallychosen so that the total voltage required to cause them to conductmatches a voltage drop that appears across the resistor 30 under somenormal load. Thus, the diodes34 initially conduct when the resistorvoltage is at some appreciable value, such as occurs when the point ofoperation on the curve A-B of FIG. 3 is in the dashed region D-B. Eventhough the switch 16 is closed, the motor armature current will flowonly through the resistor 30, and the voltage drop across the resistor30 will govern operation, so long as the wheelchair l is run on asurface and with a person that presents a relatively light load.Accordingly, the curve portion A-D represents motor operation with theresistor 30 functioning as the control element, and the curve portionD-F represents operation of the motor 6 after the diodes 34 have begunto-conduct current. The forward biased, current conducting diodes 34 nowact as a shunt around the resistor 30 to conduct a share of the armaturecurrent, and they also maintain the voltage absorbed by the parallelcircuit of resistor 30 and diodes 34 at a low level, so that armaturecurrent and motor torque rise above that which would occur if theresistor 30 were the sole voltage absorbing control element. Thetransition of the diodes 34 from a substantially nonconducting state toa conducting state is'a nonlinear function, in.which voltage changesvery little with changes in current, and this produces the curve portionD-F which deviates from the curve D-B. The addition of the diodes 34 hasthe operative effect of minimizing the presence of the resistor 30 whenlarger loads are imposed on the motor 6, and of making the voltageavailable for the armature at a higher level as load currents increase.As a result, the maximum producible torque for the motor 6 risessignificantly, as indicated by the curve D-F.

Although motor speed control might be achieved by employing only thediodes 34 in the armature circuit, without the resistor 30, use of theresistor 30 is desirable to allow limited armature current before anycounter electro-motive force is developed, and also to obtain a motortorque at normal loads with no or only a small amount of diodeconduction. This results in fewer required diodes, less heat sink forthe diodes, and therefore a more economical circuit. In design ofacontrol circuit for a wheelchair'the forward voltage drop across thediodes 34 at which conduction occurs is preferably selected to match atypical voltage drop across the resistor 30'for a normal load. This maypreferably be about 40 percent of the DC. source voltage, or of themaximum voltage that appears across the resistor 30 for a stallcondition of the motor 6. At that point the diodes will commence toconduct some of the load current, and the non-linear characteristic ofthe diodes 34 will give the rising curve portion D-F.

When desired, the speed of the wheelchair may be increased by utilizingthe high speed stage of operation. This is accomplished by depressingthe speed'control member 13 to operate the switch 17, and close thecontacts 35, 36. The lead 40 now shorts the resistor 30 and the diodes34, and the battery 21 is connected directly across the motor armature39. The entire battery voltage is available to drive the motor 6, andthe motor 6 operates on the curve A-C of FIG. 3. The use of the highspeed stage of operation may be prohibited by swinging the lever tip 19under the rod-like member 13. This would normally be done when the userof the apparatus is infirm, although there may be other reasons for notpermitting direct connection of the voltage source 21 to the motor 6.Even though direct connection may be prohibited, the diode string 34will still permit the development of substantial torque at very lowspeeds, as indicated by the point F in FIG. 3, and thus provideoperation for all but the larger loads even though direct connection ofthe battery 21 to the motor 6 is locked out.

When the manual speed control member 13 is released, and the spring 14returns it to the position shown in FIG. 2, the normally closed contact24 of the switch 15 is reconnected with the movable contact 29. Thisplaces lead 25 in the active circuit, and removes the battery 21 fromconnection with the motor 6. The lead 25 connects the resistor 30 acrossthe motor armature 39 to establish a dynamic braking circuit, which willdecelerate the wheelchair 1. By proper selection of the resistor 30, thedeceleration may be smooth without abrupt change in speed that would, orcould be upsetting to the user. It is a particular finding that aresistor 30 that functions well for controlling the speedtorquecharacteristics of the motor 6 also provides good dynamic brakingcharacteristics. Hence, only a single resistance branch and only asingle diode branch need be used for obtaining control of a permanentmagnet, fixed field DC. motor. For example, a 1/10 horsepower D.C. motoroperated from a 12 volt battery may be controlled by a one ohm droppingresistor and a train of five diodes each having a breakdown voltage ofapproximately one volt. Or, in place of a diode string a single zenerdiode with the same voltage breakdown characteristic may be selected.

The circuit breaker 22 is adjusted to open the battery tempt is made toclimb an excessively steep incline.

The location of the circuit breaker 22 is such that an opening of thebattery circuit will not interfere with the dynamic braking that occurswhen the operating member 13 is released.

Thepresent invention furnishes a motor speed control that provides notonly two speed motor control, but also an initial motor energizationstage to provide smooth wheelchair driving action. By the initialinclusion of a resistor in the armature circuit, current surge may belimited before a counter electro motive force is developed in the motorarmature, to prevent abrupt acceleration. A voltage responsive,non-linear, current conducting, gating device, such as a diode string,is brought into shunting relation with the resistor to limit the voltageabsorbed by the control circuit elements to obtain a unique lowspeed-torque relation for the motor. The circuit provides a highresistance for low loads, and automatically a low effective resistancefor the larger loads. Thus, a D.C. motor control is provided that iseconomical and that uses only few parts that are durable and troublefree in nature.

I claim:

1. In a speed control for a D.C. motor energized by a D.C. source, thecombination comprising:

a voltage dropping resistor;

first switching means connecting said resistor in circuit with the motorand the D.C. source upon operation thereof;

a voltage responsive, non-linear, current conducting,

gating device;

second switching means connecting said gating device in shunt relationto said resistor, to place said shunt connected resistor and gatingdevice in series circuit relation with the motor and the D.C. source,upon operation thereof; and

third switching means connecting the D.C. source across the motor, withsaid resistor and said gating device circumvented thereby, uponoperation thereof.

2. A speed control as in claim l,having additional switching meansconnecting said resistor across the motor in dynamic brakingrelationships upon the other switching means removing the D.C. sourcefrom connection with the motor.

3. A speed control as in claim l, wherein said gating device is a seriesof diodes having a total threshold voltage for forward conduction thematches a resistor voltage drop that occurs upon an intended normal loadon the motor.

4. in a speed control for a D.C. motor energized by a D.C. source, thecombination comprising:

a plurality of normally open switching elements;

a resistor connected in series with the motor and the D.C. source uponclosure of one of said switching elements;

a voltage stabilizing gating device electrically connected into serieswith the motor and the D.C. source, and also in parallel with saidresistor upon closure of a second one of said switching elements, saidgating device sharing motor load current with said resistor andconducting current at voltages greater than those that occur at someintended normal load condition; and

a shorting lead electrically connected around said resistor and saidgating device upon closure of a third of said switching elements. 5. Aspeed control as in claim 4, wherein said gating device is a series ofdiodes.

6. A speed control as in claim 5, wherein said series of diodes commenceto conduct at a voltage about 40 percent of the D.C. source voltage.

7. A speed control as in claim 6, wherein there is a reversing switchbetween the motor and the parallel resistor and diodes.

8. In a speed control for a D.C. motor energized by a D.C. source, thecombination comprising:

a voltage absorbing resistive element; circuit connections includingafirst switching element connecting said resistive element in series withthe motor and the D.C. source upon actuation of the switching element; I

a voltage responsive gating device that conducts upon a thresholdvoltage of approximately 40 per- 7 cent of the voltage across saidresistive element occurring upon a motor stall condition; and additionalcircuit connections including a second switching element for connectingsaid gating device in shunt relation with said resistive element tothereby position said shunt connected resistive element and gatingdevice in series circuit relation with the motor and D.C. source. 9. Ina speed control for a D.C. motor energized by a D.C. source, thecombination comprising:

a manually operable speed selector means having a switch actuatingportion movable from and to a deenergizing position; v

a plurality of normally open switching elements electrically connectedto one another and physically aligned with one alongside the other andwith operating heads in the path of travel of said switch actuationportion, which series of switching elements is electrically positionedbetween said D.C. source and the armature of said D.C. motor;

at least one resistive element electrically connected in series with thearmature of said D.C. motor upon actuation of one of said switchingelements by said speed selector means; at least one voltage responsivegating device electrically connected in parallel with said resistiveelement and serially with the motor upon the actuation of another ofsaid switching elements by said speed selector means, said gating deviceconducting and sharing load current with said resistive element upon avoltage appearing across that device that occurs for the lower values ofmotor speeds; and I a low resistance shunt line electrically connectedin parallel with said resistive element and said gating device uponactuation of still another of said switching elements.

10. A control as in claim 9, wherein said switching elements place saidresistive element in series with said motor armature upon return of saidswitch actuating portion of said speed selector means to saiddeenergizing position.

1. In a speed control for a D.C. motor energized by a D.C. source, thecombination comprising: a voltage dropping resistor; first switchingmeans connecting said resistor in circuit with the motor and the D.C.source upon operation thereof; a voltage responsive, non-linear, currentconducting, gating device; second switching means connecting said gatingdevice in shunt relation to said resistor, to place said shunt connectedresistor and gating device in series circuit relation with the motor andthe D.C. source, upon operation thereof; and third switching meansconnecting the D.C. source across the motor, with said resistor and saidgating device circumvented thereby, upon operation thereof.
 2. A speedcontrol as in claim 1, having additional switching means connecting Saidresistor across the motor in dynamic braking relationships upon theother switching means removing the D.C. source from connection with themotor.
 3. A speed control as in claim 1, wherein said gating device is aseries of diodes having a total threshold voltage for forward conductionthe matches a resistor voltage drop that occurs upon an intended normalload on the motor.
 4. In a speed control for a D.C. motor energized by aD.C. source, the combination comprising: a plurality of normally openswitching elements; a resistor connected in series with the motor andthe D.C. source upon closure of one of said switching elements; avoltage stabilizing gating device electrically connected into serieswith the motor and the D.C. source, and also in parallel with saidresistor upon closure of a second one of said switching elements, saidgating device sharing motor load current with said resistor andconducting current at voltages greater than those that occur at someintended normal load condition; and a shorting lead electricallyconnected around said resistor and said gating device upon closure of athird of said switching elements.
 5. A speed control as in claim 4,wherein said gating device is a series of diodes.
 6. A speed control asin claim 5, wherein said series of diodes commence to conduct at avoltage about 40 percent of the D.C. source voltage.
 7. A speed controlas in claim 6, wherein there is a reversing switch between the motor andthe parallel resistor and diodes.
 8. In a speed control for a D.C. motorenergized by a D.C. source, the combination comprising: a voltageabsorbing resistive element; circuit connections including a firstswitching element connecting said resistive element in series with themotor and the D.C. source upon actuation of the switching element; avoltage responsive gating device that conducts upon a threshold voltageof approximately 40 percent of the voltage across said resistive elementoccurring upon a motor stall condition; and additional circuitconnections including a second switching element for connecting saidgating device in shunt relation with said resistive element to therebyposition said shunt connected resistive element and gating device inseries circuit relation with the motor and D.C. source.
 9. In a speedcontrol for a D.C. motor energized by a D.C. source, the combinationcomprising: a manually operable speed selector means having a switchactuating portion movable from and to a deenergizing position; aplurality of normally open switching elements electrically connected toone another and physically aligned with one alongside the other and withoperating heads in the path of travel of said switch actuation portion,which series of switching elements is electrically positioned betweensaid D.C. source and the armature of said D.C. motor; at least oneresistive element electrically connected in series with the armature ofsaid D.C. motor upon actuation of one of said switching elements by saidspeed selector means; at least one voltage responsive gating deviceelectrically connected in parallel with said resistive element andserially with the motor upon the actuation of another of said switchingelements by said speed selector means, said gating device conducting andsharing load current with said resistive element upon a voltageappearing across that device that occurs for the lower values of motorspeeds; and a low resistance shunt line electrically connected inparallel with said resistive element and said gating device uponactuation of still another of said switching elements.
 10. A control asin claim 9, wherein said switching elements place said resistive elementin series with said motor armature upon return of said switch actuatingportion of said speed selector means to said deenergizing position.